This invention relates in general to the field of wafer manufacturing and, more particularly, to a method for manufacturing a phase shift photomask.
Significant advancements in the miniaturization of semiconductor integrated circuits have been made in recent years. With such advancements, a reduction in a size of circuit patterns formed on semiconductor substrates, or wafers, has been achieved. One technique for producing a circuit pattern on a wafer includes photolithography. The photolithography technique generally includes transferring a circuit pattern from a photomask onto the wafer. The photomask is generally constructed by depositing a substantially opaque layer of material on a surface of a substantially transparent substrate. Portions of the opaque layer are then removed to form the pattern to be transferred to the wafer during a light exposure step of the photolithography process.
A photomask may also be used to provide phase shifting in the photolithography process. Phase shifting generally creates positive and negative phase light interference when administering the light exposure step in the photolithography process. Exposure light reaching an unexposed region of the wafer due to optical diffraction is generally canceled out by light reaching the exposed regions of the wafer because the light transmitted through the phase shift portion of the photomask is opposite in phase. Thus, phase shifting generally provides increased resolution of transferred patterns projected onto the wafer.
Forming a photomask for use with phase shift photolithography processing generally requires reducing a thickness of the light transmitting portion of the photomask substrate to produce a desired light phase shift. Reducing the thickness of the light transmitting portion of the photomask substrate may be accomplished by etching or other suitable processes. For example, the light transmitting portion of the photomask substrate may be chemically etched until a desired thickness of the photomask substrate is obtained to produce the desired light phase shift.
However, prior methods for manufacturing a phase shift photomask suffer several disadvantages. For example, variations in etching operations generally result in imprecise thicknesses of the light transmitting portions of the photomask substrate. For example, variations in the time, temperature, RF power, and other etch process variables are generally difficult to monitor and control. Thus, the resulting light phase shift often varies from the desired light phase shift due to a deviation between the obtained photomask substrate thickness and the desired photomask substrate thickness.
Accordingly, a need has arisen for an improved phase shift photomask that provides increased control of phase shift characteristics. The present invention provides a method for manufacturing a phase shift photomask that addresses shortcomings of prior methods.
According to one embodiment of the present invention, a method for fabricating a phase shift photomask includes providing a photomask having a substantially opaque layer on a surface of a substantially transparent substrate. The opaque layer includes a removed portion to define a light transmitting pattern of the photomask. The method also includes depositing an implant in a portion of the substrate. The implanted portion of the substrate includes an etch rate different than an etch rate of an unimplanted portion of the substrate. The method includes initiating an etch of the substrate corresponding to the light transmitting pattern and monitoring a rate of the etch. The method further includes terminating the etch in response to detecting a change in the rate of the etch.
According to another embodiment of the present invention, a method for fabricating a phase shift photomask includes providing a photomask having a substantially opaque layer on a surface of a substantially transparent substrate. The opaque layer includes a removed portion to define a light transmitting pattern of the photomask. The method also includes implanting a dopant into a portion of the substrate corresponding to the light transmitting pattern. The doped portion of the substrate includes an etch rate different than an etch rate of the undoped portion of the substrate. The method also includes initiating an etch of the substrate corresponding to the light transmitting pattern and monitoring a rate of the etch. The method further includes terminating the etch in response to detecting the etch rate of the undoped portion of the substrate
The present invention provides several technical advantages. For example, the present invention provides increased control of a desired light phase shift of the photomask. For example, according to one aspect of the present invention, an implant is deposited to a predetermined depth of a portion of the substrate corresponding to a desired light phase shift. The implanted portion of the substrate has an etch rate different than an etch rate of the unimplanted portion of the substrate. An etch of the light transmitting pattern of the substrate is then initiated and monitored to detect a change in the etch rate signifying a transition of the etch from the implanted portion to the unimplanted portion of the substrate. The etch may then be terminated in response to detecting the change in the rate of the etch. Thus, the present invention provides increased control over the thickness of the substrate and the corresponding resulting light phase shift.
Other technical advantages will be readily apparent to ones skilled in the art from the following figures, descriptions, and claims.